(1) Field of the Invention
The invention relates generally to devices for discrete, small-quantity depositions of viscous material and more specifically to controlled depositions over a succession of locations. Still more specifically, the invention relates to discrete and uniform deposition of a high viscosity slurry particles in a flux material on successive locations on an electronic circuit board under computer X-Y-Z control.
(2) Description of the Prior Art
Prior art systems of the general type for effecting very small depositions of viscous fluid have been constructed conventionally, i.e., using controlled pressure or application or valving of material under pressure applied through a very small orifice (commonly a hypodermic type needle). The very small orifice through which the material must be ejected in applications such as deposition of the solder paste slurry containing particles of solder creates a practical problem in obtaining uniformity over a number of locations on the surface of an electronic circuit board. Depositions on a so-called SMD (surface mounted device) boards of small size present a unique problem in this regard. This is at least partly because of the solid solder particles passing through the small needle orifice (typically 0.02 inches in diameter) of the hypodermic needle. That small needle orifice is necessary in order to obtain a deposition on the order of 0.030 inches in diameter at each location; however, if pressure against the slurry within the syringe feeding the needle is relied upon, a relatively high pressure is required, e.g., as much as 100 psi or more. Such a pressure tends to cause separation of the solder particles from the flux acting as a fluid vehicle resulting in variation in the solder flux/ratio. Moreover, bridging or jamming of the solder particles within the needle passage can partially or completely block the flow of the slurry. A slurry which includes minute spherical solder particles is commercially available and does partially alleviate this problem; however, it is much more expensive than the commercially available slurry having random solder particles or flakes up to a nominal size in the flux.
The computer programmed servo apparatus for X-Y-Z positioning is an element of the overall combination but is known per se and has been described in the patent literature, for example, in U.S. Pat. Nos. 3,731,648 and 3,785,898.
The manner in which the invention deals with the disadvantages of the prior art by providing a novel pumping valve system capable of uniform and consistent slurry deposition will be understood as this specification proceeds.